This application claims priority under 35 U.S.C. §119 to Japanese Patent Application Nos. 2003-433505 and 2003-433506 both filed Dec. 26, 2003, the entire contents of which are hereby incorporated herein by reference.
1. Technical Field
The present invention relates to data communication performed with a communication terminal.
2. Background Information
Mobile phones having a browsing function are now widely used. Use of such phones allows a user to access a content server on the Internet and to view and download content from the server to the phone. Such mobile phones conform to either the 3G (Third-Generation) system using a W-CDMA (Wideband Code Division Multiple Access) or CDMA 2000 (Code Division Multiple Access 2000) system, or to the 2G (Second-Generation) system phones using a PDC (Personal Digital Cellular) system. Data transmission conducted using the 3G system is faster than that of the 2G system. Given that mobile phones (hereinafter, communication terminals) offer different transmission rates depending on a communication system employed by a particular terminal, if communication is performed at a transmission rate best suited to a particular communication terminal, it is likely that efficiency of data transmission for another communication terminal will be compromised.
JP 2000-278320 proposes a system for solving such a problem. Using the system, a communication terminal which receives data transmitted from a transmitter device enables an operation to be performed that ensures that data communication is conducted in an appropriate mode. Specifically, by using such a system a communication terminal is caused to measure a round trip time from when a communication message (Syn packet) requesting a start of communication is transmitted to a transmitter device until a response message (SynAck packet) is received. The communication terminal at which the data is received then estimates a communication bandwidth available in a communication path between the transmitter device, and on the basis of this determination an appropriate operation mode is employed for a communication application such as a browser. Further, the communication terminal obtains from routers on the communication path a transfer time for data transmitted between the routers, and relay devices located at each end of a link to which the decided operation mode is notified, there existing in the link a significant difference between an upload transmission rate and a download transmission rate. Consequently, the relay devices are able to set communication parameters that are optimal for the operation mode.
However, the technique disclosed in JP 2000-278320 is not necessarily effective, especially when it is applied to a communication environment such as a mobile communication network having a wireless zone where a communication state is usually unstable. In other words, a round trip time measured since transmission of a first Syn packet till receipt of a SynAck packet cannot be relied upon as a basis for determining an optimal operation mode.